In recent years, a so-called Radio Frequency IDentification (RFID) system, which is used for individual management, has been being introduced to various industries. This RFID system is a technology in which by performing radio communication between a predetermined reader/writer and a so-called transponder—a small noncontact-type Integrated Circuit (hereinafter referred to as an “IC”) device which stores various data so as to be readable and/or writable and which has a communication function, data are read from the transponder and/or written in the transponder in a non-contact manner. Specifically, the RFID system is based on the principle of electromagnetic induction, in which the magnetic flux generated from a reader/writer's loop antenna is magnetically coupled to a transponder's loop antenna through inductive coupling, whereby communication is performed between the transponder and the reader/writer. The RFID system is promising in various applications. For example, a transponder fabricated in the form of an IC tag is attached to articles for manufacturing management and/or logistics; a transponder fabricated in the form of an IC card is used for fare collection in transportation facilities or as an identification card used when someone enters or leaves buildings; and a transponder is mounted in, for example, portable phones, which then serves as an electronic money card for purchasing articles.
Such RFID system can write in or read out data easily and rapidly. This is because, unlike conventional contact-type IC card systems, the IC card based on the RFID system does not involve inserting into its reader/writer or contacting its metallic contact. Further, since necessary electric power is supplied to a transponder from a reader/writer through electromagnetic induction in the RFID system, a power source (e.g., a battery cell) is not required to be built it. Thus, the RFID system is advantageous in that it realizes provision of a transponder which has a simple structure, which is supplied at low cost, and which has high reliability.
In the RFID system, when there are metallic objects around a transponder, communication failures may occur due to them. For example, when mounted in a portable communication device such as a portable phone, a transponder is adversely affected by a metal casing or metal parts of the portable communication device to cause a problem in that the communication distance is shortened. The reason for this is as follows. Specifically, when a metallic object exists around a transponder based on the electromagnetic induction system, unfavorable displacement of resonance frequency, change in magnetic flux, etc. are caused by a change in inductance due to the metallic objects, resulting in that sufficient electric power cannot be ensured. Therefore, in the RFID system, in order to ensure a sufficiently broad range where a transponder can communicate with a reader/writer to a satisfactory extent, the transponder must be provided with a loop antenna which can radiate a electromagnetic field of a certain intensity of magnetic field.
In this case, in order that the adverse effects to the loop antenna by the metallic objects are reduced in other ways than a way in which the spacial configuration is appropriately changed, for example, use of magnetic materials is effective and reduces the adverse effects due to the metallic objects, lengthening the communication distance. Meanwhile, in the recent communication or electronic devices, a noise electromagnetic wave is more frequently radiated as the clock frequency becomes higher. As a result, external or internal interference occurs to cause, for example, improper operation of the devices themselves, adverse effects to peripheral devices, etc. The aforementioned magnetic materials are effective for preventing such electromagnetic interference. In view of this, for example, proposed are various composite magnetic sheets (soft magnetic sheets) which are prepared by dispersing/mixing an appropriate amount of flat soft magnetic powder in a binding agent such as rubber or plastic. When a transponder is mounted in a portable communication device such as a portable phone, in order to prevent the communication distance from being shortened due to a metal casing or battery pack of the portable communication device, a magnetic material sheet is attached to enhance the convergence of magnetic flux in an attempt to realize stable communication (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2005-333244).
Meanwhile, in recent years, portable communication devices (e.g., portable phones) have been being made thinner, lighter and cheaper. In view of this trend, various constituent parts of the portable communication devices are required to be made thinner and lighter.
Here, a battery pack is large of the constituent parts of each portable communication device. Thus, as shown in FIG. 10, for example, when an antenna main body 102 is disposed so as to surround the side wall of a battery pack 101 whose cross-sectional surface is generally rectangular, a large loop antenna can be formed and its structure can be made thinner than a conventional structure where a magnetic sheet and a loop antenna are disposed on the back surface of a battery pack.
In this structure, a magnetic sheet 103, which is formed so as to have an elongated shape whose width is virtually equal to the thickness of the battery pack 101, is attached via a predetermined adhesive material 104 so as to be along an antenna main body 102. When the magnetic sheet 103 is attached to the antenna main body 102 as described above, as indicated by reference character A in FIG. 10, the portions thereof corresponding to, for example, the corners of the battery pack 101 are folded.
The magnetic sheet 103 attached to the antenna main body 102 is, for example, exposed to a considerable amount of heat generated during charging of the battery pack 101, or to a considerable change in temperature caused when a user takes it in and out a pocket of his/her wear. In the portable communication device, when the magnetic sheet 103 is alternatingly exposed under low-temperature conditions and under high-temperature conditions or continuously exposed under high-temperature conditions, unfavorable deflection or floating between the magnetic sheet 103 and the antenna main body 102 may occur caused by the difference in linear expansion coefficient between the magnetic sheet 103 and the antenna main body 102. As a result, the portable communication device containing such a magnetic sheet involves unfavorable displacement of resonance frequency due to such deflection or floating. In view of this, demand has arisen for a portable communication device whose characteristics are not changed depending on its surrounding environment.
Here, by increasing the amount of the flat magnetic powder (i.e., a main material) contained in the magnetic sheet 103, its linear expansion coefficient can be decreased and made close to the linear expansion coefficient of the antenna main body 102 (for example, 17 ppm/° C. in the case of the antenna main body made of copper). However, when the amount of the flat magnetic powder contained in the magnetic sheet 103 is intended to be increased, its linear expansion coefficient may not be decreased depending on the type of a binder used. Further, when the magnetic sheet 103 is decreased in linear expansion coefficient, the flexibility thereof is decreased. As a result, when folded, the magnetic sheet involves failures such as crack formation. Such failures cause a problem in that the magnetic powder is diffused in the portable communication device, leading to a drop of reliability; e.g., degradation of performance.
In addition, as the amount of the flat magnetic powder contained in the magnetic sheet 103 is increased, the magnetic sheet becomes brittle. As a result, when alternatingly exposed under low-temperature conditions and under high-temperature conditions or continuously exposed under high-temperature conditions, the magnetic sheet problematically changes in thickness and/or magnetic characteristics. In contrast, when the amount of the flat magnetic powder contained in the magnetic sheet 103 is small, sufficient magnetic characteristics cannot be obtained, although the occurrence of the failures can be reduced. As a result, the communication distance of the formed portable communication device is shortened, which is problematic.
A so-called coating method, which is one method for producing a sheet of a soft magnetic composition, is suitable for the production of a thin magnetic sheet. But, for producing a thick magnetic sheet, it is necessary that a plurality of magnetic sheets are laminated and pressed with a laminator or pressing machine. Here, for decreasing the linear expansion coefficient, when a magnetic sheet is formed by laminating a plurality of magnetic sheets each of which is made using an epoxy group-containing acrylic rubber, an epoxy resin, an epoxy curing agent and a flat magnetic powder, it involves cracks to the same extent when folded both so that the front surface is folded inward and so that the back surface is folded inward; i.e., has low impact resistance.
In one possible solution against the problems relating to the amount of the flat magnetic powder, a magnetic sheet in which the relative flat magnetic powder amount to the binder is large is attached to another magnetic sheet in which the relative flat magnetic powder amount to the binder is small. The thus-obtained magnetic sheet is prevented from crack formation occurring when folded either so that the front surface is folded inward or so that the back surface is folded inward. The magnetic sheet having such a structure is very difficult to attain desired magnetic characteristics.